The use of digital computers within commercial aircraft systems has expanded the scope of automated maintenance-related monitoring and testing, and has improved the procedures for fault isolation. Fault isolation refers to the maintenance procedure for determining, from crew observations and/or testing, components that require replacement or repair. Maintenance procedures include the isolation of faults as well as the determination of the significance of a fault for maintenance scheduling. The traditional practice of fault isolation is based on flight crew observations and maintenance personnel analysis using maintenance manuals and operator initiated testing. This practice is highly dependent upon crew observation, memory and analysis, and on the timely transfer of data to the maintenance personnel.
It is customary to categorize aircraft maintenance activities into unscheduled and scheduled maintenance. Unscheduled maintenance is performed as required to maintain the aircraft's airworthiness during intervals between scheduled maintenance. Unscheduled maintenance is usually performed while the aircraft is on the ground between flights, although longer periods of time may be required, often resulting in schedule disruptions. Minimum ground time between flights is desirable to maximize airplane utilization; therefore, the time allocated is often limited to that required to refuel and service the aircraft, typically 20 to 120 minutes. Ground crews, therefore, need complete and accurate information regarding any repairs that will be required. The information must be made available as soon as possible to allow for preplanning of activities. The information must be precise so that a minimum amount of time is spent troubleshooting while the aircraft is on the ground in order to find the source of a problem.
Scheduled maintenance is comprised of specific tests, inspections, and repairs that are performed at predetermined intervals. These events are scheduled in advance and therefore rarely result in aircraft schedule interruptions.
To facilitate both scheduled and unscheduled maintenance, most aircraft systems are made up in part of components that can be removed and replaced quickly. These components are called Line Replaceable Units (LRUs). An LRU may be mechanical, such as a valve or pump; electrical, such as a switch or relay; or electronic, such as an autopilot or an inertial reference computer.
In the latest generation of aircraft, most electronic LRUs are actually digital computers. These computers are responsible for the control and monitoring of nearly all the various aircraft systems. The use of digital computers makes possible a degree of fault monitoring and semiautomatic system testing that was not previously possible. This fault monitoring and semiautomatic system testing is commonly referred to as Built in Test Equipment (BITE).
Other aircraft systems do not rely on BITE but may be monitored and tested in other ways, often through a scheduled maintenance task. An example of this is an anti-ice system that is controlled by a simple switch/relay network. Still other systems are not monitore electronically in any way. These are referred to as "nonmonitored systems". These systems rely on scheduled inspection or testing to determine maintenance requirements. Examples of nonmonitored systems are interior components such as seats or the aircraft structure itself.
BITE is software and hardware integrated into a computerized control system. The BITE is programmed to monitor the connected or attached LRUs as well as the control system itself. BITE diagnostics programs are run periodically either interactively or automatically to detect faults and to generate messages indicative of operating conditions. Generally, specific words or bits of words are set to communicate operating conditions. The testing and fault analysis performed by BITE may be initiated independently of operation of other LRUs and aircraft components.
In a monitored non-BITE system, no computerized control system including BITE is directly connected to the LRU. These LRUs are generally made up of a series of valves, lines, switches. etc. The LRU has certain operational characteristics that are monitored and tested for maintenance purposes by tapping into the system's discrete wiring. The characteristics that are monitored and tested include open/closed circuitry, voltage level, etc. This type of data is referred to as analog discrete data to distinguish it from the digital discrete and analog data generated by BITE.
The LRU in a non-BITE system is generally tested by maintenance personnel who utilize physical measurement testing devices to ascertain the state of the LRU. This maintenance information related to the LRU is not collected on a continuous basis. If a fault occurs in the LRU, the fault is usually observed by a flight crew member or maintenance personnel or is indicated during standard testing. If a fault is observed, then it is identified by utilizing manual test equipment and a maintenance manual. These procedures are extremely time consuming, depend upon observations and regular testing, and require the participation of experienced operators.
A main goal of aircraft maintenance procedures is to identify and correct LRU faults by monitoring their operation in flight and analyzing the flight data in order to isolate the LRU fault. The fault isolation procedure requires the consolidation of fault data in order to screen out faults that are reported by multiple LRUs and to reduce the fault indication down to the fault source, i.e., a specific component, LRU, or computerized control system. Automated or standardized methods of consolidation have been developed to aid maintenance personnel. Otherwise, the maintenance personnel would be required to consider each piece of data generated during flight. In order to facilitate aircraft maintenance procedures, it is desirable to automatically monitor a complete set of LRUs in flight, to accurately and quickly identify specific LRU faults from data obtained during the flight and from minimal additional testing, and to test replaced or repaired units.
As the number of aircraft systems that make use of digital computers with BITE has grown, so has the volume of maintenance information and the number of different ways the information is presented. The volume and presentation make it difficult for ground crews to obtain the specific information they need to effect the required repairs--hence, the need for a system that will collect and consolidate all of this information.
A number of maintenance systems have been developed that attempt to meet these maintenance procedure goals. Many of the systems are based on a centralized scheme for fault data collection and analysis. In these centralized systems, LRU data from multiple LRUs is automatically collected during flight or at the end of the flight and analyzed by the maintenance personnel. Some systems also provide automated fault isolation procedures. Some of the major drawbacks of the previous centralized systems include: lack of uniformity in the centralized system and LRU fault reporting schemes; lack of an overall aircraft view for isolating faults; and inability to collect information from a complete set of LRUs. For these reasons, many maintenance systems still require extensive operator observation and intervention for fault isolation. This reliance on flight crew and maintenance personnel can result in time consuming and inaccurate maintenance procedures.
Centralized maintenance systems generally include data collection, data processing, data display, and operator command input components. In one type of system, prior to the inclusion of separate BITE in computerized control systems, a centralized BITE scheme was used wherein fault information related to an LRU was encoded by the LRU itself and transmitted over a serial data bus to a central control system. A number of LRUs were connected to the central control system. The central control system received and presented the fault data to the operator in much the same format as it was transmitted by each LRU. Thus, to understand the data transmitted from various LRUs, the operator was required to be familiar with the message format and operation of each LRU. The main task of the central control system was to display the individual LRU fault data. Fault isolation procedures were generally not feasible because of the nonuniform formats of the data. This scheme is still used by many aircraft systems to collect and report information for groups of related LRUs.
In these centralized BITE schemes, the fault data received at a central control system might be prioritized and displayed during flight in the form of flight crew observations. Faults may be prioritized by whether crew awareness or action is required. The observations appear as indications on a dedicated display such as the primary flight display. In some aircraft systems, it is the flight crew's responsibility to record the observations along with other operating parameters such as time of occurrence. These observations and operating parameters are then passed on to the maintenance personnel upon landing. Generally, a series of further automated or manual fault isolation tests must be performed in order to locate the fault. This type of maintenance system relies heavily on the accuracy of the flight crew observations. If an observation is missed or is incorrect then the isolation of a fault may be extremely time consuming or may not be possible from the information provided. Additionally, since not all faults are observed and/or recorded, much LRU flight data is not available to the maintenance personnel.
Alternatively, the central control system stores the LRU fault data generated during a flight. The data may be obtained from the LRUs continuously in flight or may be gathered at the end of each flight and transmitted to the maintenance personnel. The central control system may additionally correlate the observations or faults with operating parameters such as time of occurrence. In this system, although a great deal of LRU data may be provided to the maintenance personnel, the data may be of little help because it may be made up of cryptic notations, It may include nonfault or irrelevant information, and, because there may be no uniformity of format, the data as reported may not be suitable for analysis by automated means.
With the provision of a variety of computerized control systems in the overall aircraft system, BITE has been included directly in the control system computers. Thus, a centralized control system may receive LRU data from BITE rather than each LRU. Problems of crew understanding and data manipulation still exist because of the nonuniformity of data collected from BITE. Certain steps have been taken for making BITE test initiation commands and test result formats uniform to aid in inter-control system communications. For example, ASCII characters in an Aeronautical Radio Inc. (ARINC) 429 format are widely provided by BITE systems. This format produces English type messages.
A distributed or a federated BITE system is a central control system responsible for a series of computerized control systems which each include BITE. The distributed BITE system may include a central computer that acts as a passthrough device to collect and display the English type messages generated by each BITE. Further, the display system may provide means for operator initiated testing of specific systems by accepting a test initiation command and forwarding the command to the BITE. The test results are received from the BITE and passed through to the operator. One drawback of the distributed BITE system is that the operator is still required to have an understanding of characteristics of specific BITE in formatting the test initiation commands and analyzing the test results. Because the message formats are BITE specific, it is difficult to automatically analyze a message in conjunction with messages received from independent BITE in order to isolate faults. The English-type messages are meant to be directly observed and understood by flight crew and maintenance personnel rather than automatically combined with other messages to produce an isolated fault message. Additionally, some control systems generate coded messages wherein each message is a series of symbols that correspond to a message in a maintenance manual. Thus, in order to utilize the message provided by the central control system, the operator must have access to maintenance manuals or a keen knowledge of the specific system.
The nonuniformity of fault data formats generated by BITE reduces the utility of collecting the in flight fault data for an entire aircraft system in a centralized system. Since the maintenance personnel cannot manipulate the fault data because of the nonuniformity, there is no reason to retain in flight data in excess of the observation reported to the flight crew and a small number of fault data indications that are considered to be of high priority.
Some central-type control systems monitor a set of computerized control systems that are related to a specific aircraft function. For example, a central-type control system may monitor only BITE related to autopilot and flight director functions. The identified BITE can then be customized to produce uniform messages. By requiring uniform messages to be generated by each BITE and having an understanding of the operation of the limited systems being monitored, some fault isolation may be performed by the central-type control system. In many instances, only selected fault data that is deemed necessary to carry out the specific function is transmitted from the BITE to the central-type control system. This limited data collection is based on limitations in data storage, hardware connections, etc. Additionally, the fault isolation procedure is generally limited to generating system or LRU level fault reports. This is the case since further analysis, i.e., to generate shop faults, requires a great deal of information about the system components' interconnection and fault status.
One of the early applications of digital computers with BITE was the autoflight system for The Boeing Company's 757 and 767 aircraft. The autoflight system is made up of multiple digital computers connected to various sensors and actuators. The computers monitor themselves, the sensors and actuators, and the other computers and report the results to a single maintenance control and display panel (MCDP) computer. This computer performs additional fault isolation if necessary and presents this information as messages to the ground crew indicating which specific LRU or interfaces between LRUs have failed. The computer also indicates what, if anything, the flight crew observed as a result of the fault. The MCDP computer also allows the ground crew to initiate a set of semiautomatic tests covering various parts of the autoflight system. These tests help further isolate problems as well as allowing the verification of proper LRU installation following a replacement. The autoflight system monitors a limited set of information. Only the flight control, thrust management, and flight management computers are connected to the MCDP computer. Additionally, only certain predetermined faults are recorded by the MCDP computer. Finally, only limited data analysis is performed by the MCDP computer. The analysis is generally meant to reconcile conflicting fault messages received from the monitored computers. Otherwise, the fault analysis is done separately by each monitored computer.
The central control systems described above generally suffer from the lack of ability to provide maintenance data consolidation and aircraft-wide fault isolation. Even if all of the BITE information is in a uniform format, i.e., an English-type format, the messages are not easily combinable to facilitate fault analysis. There is the additional consideration that not all LRUs are associated with BITE. Therefore, a complete set of LRU fault data is a superset of the BITE fault data. A centralized fault handling system for analyzing a complete set of LRU fault data requires that the data to be in a uniform combinable format and that there be a thorough understanding of the overall aircraft system interconnections and operation.
The present invention overcomes these and other problems in the prior art.